Ultraviolet light curing compositions for composite repair

ABSTRACT

An ultraviolet (UV) light curable formulation useful for repairing composite materials, comprising: an acrylic oligomer, an acrylic monomer, and a photoinitiator. This formulation may include fiberglass. The photoinitiator can be a combination of a bis-acylphosphine oxide and an alpha hydroxy ketone. The formulation can cure rapidly, such as in about 20 minutes. The cured formulation can have a T g  above 150° C.

This application is a divisional of patent application Ser. No.10/784,002, filed Feb. 20, 2004, now U.S. Pat. No. 7,144,544, whichclaims priority to U.S. Provisional Application Ser. No. 60/448,587,filed Feb. 20, 2003, each of which is incorporated herein by reference.

Subject to right of the assignee afforded under a Small BusinessInnovation Research (SBIR) program and SBIR Project AF01-131, the U.S.government has a paid-up license in this invention and the right inlimited circumstances to require the patent owner to license others onreasonable terms as provided for by the terms of contract numberF33615-02-C-5605 which was supported by The United States Air ForceResearch Laboratory at Wright-Patterson Air Force Base.

BACKGROUND OF INVENTION

This invention pertains to a UV light curable composition which maycomprise an acrylic oligomer, an acrylate monomer, a photoinitiator, anda filler such as fiberglass.

Rapid, high-quality, on-aircraft repair techniques for fiberglasscomposite components are desirably. However, current field level repairtechniques use thermally accelerated adhesive bonding to restore theoriginal design strength of the composite laminate. In practice, severalproblems exist with on-aircraft thermally cured repair methods. Airframestructural members act as heat sinks, and make it difficult to obtain auniform cure temperature profiles. Excessive power requirements canresult from efforts to offset this heat sinking effect. Also, commonlyused resin systems require low temperature storage to avoid prematuredegradation, increasing storage cost and support complexity.Furthermore, the thermally accelerated require heat blankets for curethat can be difficult to work with depending on the size and geometry ofthe aircraft part being repaired.

SUMMARY OF INVENTION

The present invention provides a solution to one or more of the problemsand deficiencies in the prior art. For example, this invention providesan ultraviolet light curable resin system that can nullify the problemsand deficiencies identified above while retaining the necessary strengthand adhesion requirements for a composite repair.

In one broad respect, this invention is an ultraviolet (“UV”) lightcurable composition useful for repairing composite materials,comprising: an acrylic oligomer, an acrylic monomer, a photoinitiator,and fiberglass. In one embodiment, the composition comprises about 10 toabout 50 percent by weight of the acrylic oligomer, about 20 to about 60percent by weight of the acrylic monomer, about 0.5 to about 3 percentby weight of the photoinitiator, and about 25 to about 75 percent byweight of the fiberglass; comprises a composition wherein the whereinthe photoinitiator is a combination of a bis-acylphosphine oxide and analpha hydroxy ketone; comprises a composition wherein the photoinitiatoris a combination of a bis-acylphosphine oxide and an alpha hydroxyketone, and wherein the bis-acylphosphine oxide to alpha hydroxy ketoneratio is from about 1:4 to about 4:1; or combination thereof.

In another broad respect, this invention is a UV curable formulation,comprising: an acrylate oligomer, a combination of two or more acrylicmonomers, a bis-acylphosphine oxide, an alpha hydroxy ketone, andoptionally fiberglass, wherein the cured formulation formed from thecurable formulation has a T_(g) greater than 150° C.

In another broad respect, this invention is a reaction product formed byirradiation of the UV curable composition or the UV curable formulation.

In another broad respect, this invention is a method which comprises:combining an acrylic oligomer, an acrylic monomer, and a photoinitiator;applying the resulting UV curable formulation to fiberglass to therebyform a UV curable formulation. This method may further comprise applyingthe UV curable formulation onto a fiberglass layer to form a UV curablecomposition. The method may also comprise curing the UV curablecomposition using UV irradiation to form a cured composition. Thealternating layers of fiberglass and the uncured or cured composition ofthis invention form a composite material (a laminate), wherein the voidsin the fiberglass may have been wetted out by application of the uncuredcomposition.

In another broad respect, this invention is a method of repairing a holein the exterior of an airplane, comprising: applying alternating layersof a UV curable formulation and woven fiberglass fabric to fill the holeand to form a UV curable composition; creating a vacuum across at leastone side of the UV curable composition; irradiating the UV curableformulation with UV light to cure the formulation to produce a curedcomposition; and removing the vacuum. The vacuum can be applied usingknown, conventional procedures. Similarly, the UV radiation can besupplied with conventional equipment and depending on the UV curablecomposition can be effected by sunlight. The hole to be repaired can beof a variety of depths and widths. In general the width can be up to twofeet and typically up to one foot, and the depth can be up to about 200mils (0.2 inch), typically up to about 150 mils (0.15 inch), andtypically in the range from about 10 to about 150 mils. It should beappreciated that the hole or damaged area can be partially through agiven composite piece to be repaired or can be completely through thepiece such as in the case of a hole through a portion of a wing orfuselage. The damaged area is typically damage to the exterior skin ofthe composite, though portions of the core may also be repaired usingthe UV curable composition. It should also be noted that the shape ofthe portion of the composite to be repaired can be of essentially anyshape, and vary widely depending on how the damage to the compositematerial of the aircraft or other structure built of a compositematerial is damaged. In one embodiment, the repair can be performed onstructures formed of Nomex honeycomb cores with thin skins, such as astructure with an aluminum core and aluminum alloy skins or an aluminumcore with fiberglass-reinforced or carbon fiber-reinforced epoxy skins(sometimes referred to as having, for example, fiberglass facings). Thematerials to be repaired in accordance with repaired are compositematerials, such as those made of using a honeycomb structure and/or madeof carbon composite materials. The hole to be repaired is at leastpartially filled with the curable composition and then cured with UVradiation. In some cases it may be desirable or necessary to remove askin layer so that the UV curable composition or UV curable formulationmay be applied to the core rather than to an epoxy skin, so as toincrease bonding and/or to enable the repair patch to have the sameheight as the skin; that is, to provide a repair area that issubstantially similar in depth to the original epoxy skin. Afterexposing an additional portion of the core by removing a portion of theundamaged skin, a thin layer of UV curable formulation is applied, uponwhich alternating layers of fiberglass and UV curable formulation areapplied (the top layer is UV curable formulation) to form the UV curablecomposition (i.e., a composite) having alternating layers of fiberglassand UV curable formulation. It may be desirable to at least partiallyshade the area where the layers are applied so that ambient UV lightdoes not prematurely initiate cure. A vacuum bagging procedure, wellknown to one of skill in the art, may be employed to reduce the amountof bubbles in the final cured composition. A bagging procedure mayinclude a number of layers of material over the UV curable composition,such as a layer of Teflon film, a layer of fiberglass, a perforatedTeflon layer, fiberglass cloth, non-porous Nylon 66 separator film, alayer of breather cloth, and the vacuum bag to which is attached thevacuum source. Typically the vacuum bagging is performed at ambienttemperatures. Similarly, the area may be tamped prior to irradiation toremove at least a portion of trapped air bubbles. In one embodiment, thevacuum is maintained during UV curing.

The UV cure resins of this invention do not require heating. Also, longambient temperature storage is possible with the present invention. Bythe practice of this invention, cure times can be significantly reducedrelative to current methods, thus increasing aircraft availability andreducing repair cost.

The main problem contemplated by the inventors was that for a UV curedapproach, the difficulty is in developing a resin system with sufficienthigh temperature tolerance. In addition, it would be desirable for someapplications if the methods for on-aircraft cure of UV resins matchedthe results of thermally cured resins in terms of uniform physicalproperties. The inventors herein sought to provide a UV cure repairsolution that includes resin systems, reinforcements, vacuum baggingmaterials, UV illumination sources, and detailed repair procedures.

This invention provides a number of advantages over the current epoxyresin products and these advantages are detailed below. These advantagesinclude: quick cure times on the order of minutes instead of hours;sunlight-only cures are possible; no frozen storage required; simple toapply such as being similar to ordinary epoxy resin wet layup systems;ability to cure through relatively thick bagging schedules, as long asno UV radiation blocking layer(s) are used; only a mild acrylic odorpresent and no styrene emission problems; easy to clean up with commonsolvents; no expensive and fragile hot bonders needed for field levelrepairs; no programming of hot bonders, with knowledge of specific ramprates, soak times, alarm thresholds, etc., needed by the repairtechnician; no possibility of a runaway heat blanket with attendant firerisk; no thermocouples required; and the strength of cured resins aresimilar to conventional epoxies.

DETAILED DESCRIPTION OF THE INVENTION

As discussed above the formulations of this invention include one ormore oligomers, one or more monomers, one or more photoinitiators, andone or more fillers. Aircraft may be repaired using standard vacuumbagging procedures, which reduces the amount of voids and detrimentaleffects of oxygen during cure. Standard UV lighting equipment may beemployed. Several layers may be built up and simultaneously irradiatedto effect curing. The formulations herein have the unique and surprisingbenefit of being able to fully cure despite having several layers opaqueto visible light.

Oligomers

The acrylated oligomers that may be used in this invention can varywidely, and may include a variety of backbone structures in addition tothe acrylate moiety, such as urethane, epoxy, and polyesterfunctionality. An oligomer is generally referred to as a polymeric unitcontaining two to four, possibly more, monomer units. An oligomer istypically composed of only a few monomer units such as a dimer, trimer,tetramer, etc., or their mixtures. The upper limit of repeating units inan oligomer is usually considered to be about twenty. The term telomeris sometimes used synonymously with oligomer. Oligomers are typicallyhigher molecular weight (1,000-30,000 g/mol) crosslinkable coatingcomponents used as the base material in a coatings formulation. Theprimary job of the oligomer is to impart the major physical propertiesof the finished coating. The oligomers employed in this invention arebased on a variety of chemistries, including acrylated urethanes,epoxies, polyesters and acrylics. The acrylated oligomers used in UV/EBradical polymerization are typically viscous liquids ranging from a fewthousand centipoise to greater than one million centipoise in viscosityat 25 C. The acrylated oligomers typically possess two to six acrylategroups per molecule and range in molecular weight from approximately 500to 20,000 g/mol.

In acrylate chemistry there are several families of oligomers. Eachparticular family has both advantages and disadvantages. The primaryoligomer families are generally referred to as epoxy acrylates, urethaneacrylates, polyester acrylates, polyether acrylates, amine modifiedpolyether acrylates, and acrylic acrylates.

A representative sample of suitable acrylates is provided in Table 1.

TABLE 1 Various Acrylic Oligomers for Resin Formulation Tradename TypeManufacturer T_(g) (° C.) CN975 Hexafunctional urethane Sartomer 29acrylate CN104 Epoxy acrylate Sartomer 67 CN120 Epoxy acrylate Sartomer60 CN151 Epoxy methacrylate Sartomer 68 BR-941 Hexafunctional aliphaticBomar 83 urethane acrylate BR-970 Aliphatic urethane acrylate Bomar N/ABR-990 Trifunctional urethane Bomar 20 acrylate Genomer 4302 Aliphaticpolyester Rahn 40 triurethane triacrylate Genomer 2252 Acrylatedbisphenol A Rahn N/A epoxy resin

Epoxy acrylate oligomers impart high gloss, hardness, fast cure, pigmentwetting and chemical resistance to coatings. As with the use ofmonomers, molecular weight, functionality and chemical nature of theepoxy acrylate also allow variability within the same class ofmaterials. Urethane acrylate oligomers provide excellent weatherabilityin the case of aliphatic products, as well as abrasion resistance,scratch resistance, impact resistance and flexibility.

One of the principal roles of the oligomer species is to promoteadhesion of the resin to the fiberglass of the composite, as well asincrease the tensile strength and toughness by reducing brittleness. Assuch, choice of the oligomer is important in the practice of thisinvention. Typically, these properties are achieved at the expense ofhaving lower (<50° C.) glass transition temperatures (T_(g)). Acrylatedoligomers used in this invention generally form cured compositions thathave a Tg in the range of 100 to 175° C. This invention provides acomposition (a cured composite) that will withstand excess temperatures(>150° C., as this is the upper service temperature, e.g. 175° C.).

Aromatic difunctional epoxy acrylate oligomers may be used in oneembodiment of this invention, and in one respect difunctional epoxyacrylate oligomers derived from bisphenol A may be employed. This typeof oligomer has very low molecular weight that gives them some verydesirable properties including, high reactivity, high gloss, high glasstransition, high strength, and low physiological irritation. The cost ofthese products is very low. This makes these types of oligomers suitablefor a wide variety of applications, ranging from overprint varnishes forpaper and board to wood coatings for furniture and parquet flooring, butalso high tech applications like compact disk coatings and optical fibercoatings. Their main drawbacks are high viscosity, some long-termyellowing, and limited flexibility. Because of this, they are lesssuitable for application on flexible substrates; low viscosityapplication techniques like spray-, dip-, curtain coating, andapplications with high requirements in terms of color stability over alonger period of time (white and light colored substrates that have tolast long).

In the absence of fiberglass, the UV curable formulations (oligomers,monomers, and photoinitiators) generally have an amount of acrylicoligomer in the range from about 20 to about 70 percent by weight, andin one embodiment is an amount in the range from about 20 to 60 percentby weight. As used herein, “UV curable formulation” or “curable resin”refer to a formulation containing oligomers, monomers, andphotoinitiators, but which does not contain fiberglass. By contrast, asused herein a UV curable composition refers to a combination of theformulation with fiberglass, such as by layering formulation andfiberglass sheets to form monolithic structures that can includemultiple layers of fiberglass sheets.

Monomers

In order to raise the glass transition temperatures of the curedcomposite resins, the aforementioned oligomers must be successfullycopolymerized with one or more monomers known to have high T_(g)'s (suchas those listed in Table 2), resulting in an overall resin system with ahigh T_(g) while retaining the necessary toughness that is desired forcomposite strength. The monomers used in this invention are typicallycapable of raising the T_(g) of the cured resin to above 150° C.

The acrylic monomers used in this invention can be monofunctional,difunctional, and trifunctional acrylic, acrylate and methacrylatemonomers. Representative examples of such monomers include but are notlimited to: methyl methacrylate (MMA), ethyl methacrylate, methacrylicacid (MA), isobornyl methacrylate (ISBM), ethylene glycol dimethacrylate(EGDM), ethoxylated bisphenol A diacrylate esters (BPADAE),tetraethylene glycol dimethacrylate (TEGDM), diethylene glycoldimethacrylate (DEGDM), diethylene glycol diacrylate (DEGDA),tris(2-hydroxyethyl) isocyanurate triacrylate (ISOTRI) as well as thediacrylate, alkyl (such as isodecyl, butyl, methyl, tetrahydrofurfuryl,and 2-ethylhexyl) or hydroxy alkyl (such as hydroxy ethyl and hydroxypropyl) esters of acrylic acid and methacrylic acid, butyleneglycoldiacrylate and triacrylate, 1,6-hexanediol diacrylate,tetraethyleneglycol diacrylate and triacrylate, polyethylene glycoldiacrylate and triacrylate, bisphenol A diacrylate and triacrylate,pentaerythritol diacrylate and triacrylate and tetraacrylate; alkyl andhydroxyalkyl acrylates and methacrylates, e.g. methyl, ethyl, butyl,2-ethylhexyl and 2-hydroxyethyl acrylate, isobornyl acrylate, ethyleneglycol diacrylate, propylene glycol diacrylate, neopentyl glycoldiacrylate, hexamethylene glycol diacrylate,4,4′-bis(2-acryloyloxyethoxy)diphenylpropane, trimethylolpropanetriacrylate, and vinyl acrylate. Combinations of these monomers may alsobe employed. Likewise, one or more of these monomers may be excluded.Other monomers may be included in the curable composition of thisinvention depending on the end use and desired properties of the curedresin.

TABLE 2 Acrylic Monomers for Resin Formulation Component TypeManufacturer T_(g) (° C.) SR 368 Tris(2-hydroxyethyl) Sartomer 272isocyanurate triacrylate SR 423 Isobornyl methacrylate Sartomer 110Genomer 1223 1,6 Hexanediol diacrylate Rahn 43 SR 444 Pentaerythritoltriacrylate Sartomer 103 Methacrylic acid Aldrich 216 Methylmethacrylate Aldrich 105

Monomers are used as reactive diluents in some formulations. Monomerscan also be used to achieve a number of desired properties includingglass transition, adhesion, reactivity, chemical resistance, scratchresistance, and strength. Thus, selection of a given monomer can dependon one or more of these criteria. A higher amount of functionality ofthe monomer results in higher reactivity. A lower amount offunctionality results in lower shrinkage and better adhesion. Generallythe lower the molecular weight the lower the viscosity. Combinations ofmonomers can be used in the practice of this invention to achievedesired final properties of the cured resin.

In the absence of fiberglass, the UV curable formulations (oligomers,monomers, and photoinitiators) generally have an amount of one or moreacrylic oligomers in the range from about 20 to about 90 percent byweight, and in one embodiment is an amount in the range from about 30 toabout 80 percent by weight, and in a second embodiment in the range fromabout 40 to about 75 percent by weight. Typically, two or more monomersare employed; and in one embodiment the two or more monomers are acombination of ISOTRI, ISBM, MMA, and MA.

Photoinitiators

Photoinitiators are chemicals that form energetic radical species whenexposed to UV light. They are essential ingredients in UV-curablecoatings in order to obtain polymerization. Depending on factors such asfilm thickness, UV-light source and particular coating performancerequirements, the amount of photoinitiator in a UV-coating formulationcan range from approximately 0.5 to 15%. Photoinitiator systems areavailable that meet the particular requirements for curing very thinclear coatings, thin pigmented coatings, and very thick clear coatings.

Representative photoinitiators include but are not limited to thoselisted in Table 3. A particularly effective system in the practice ofthis invention is a 3:1 ratio of an acylphosphine (such asphenylbis(2,4,6-trimethylbenzoyl)-phenylphosphineoxide, availablecommercially as Irgacure 819) to phenyl ketone (such as1-hydroxy-cyclohexyl-phenyl ketone, available commercially as Irgacure184), which has shown to be very effective in thoroughly curingcomposite samples, even those approaching 5.0 mm in thickness.Photoinitiator concentrations typically range from about 0.5 to about3.0 percent by weight of the UV curable formulation. Thebis-acylphosphine oxide and the α-hydroxy ketone combination ofphotoinitiators were found to be very effective because of their abilityto initiate cure in thick sections of a composite formed from resin andfiberglass layers.

TABLE 3 Various Photoinitiators for Resin Formulation Initiator TypeManufacturer Irgacure 819 Acylphosphine oxide^(a) Ciba Irgacure 184Phenyl ketone Ciba Irgacure 2020 Mixture of a phosphine Ciba oxide and ahydroxy ketone^(b) ITX Benzophenone derivative First Chemical^(a)Irgacure 819 isphenylbis(2,4,6-trimethylbenzoyl)-phenylphosphineoxide. ^(b)Irgacure2020 is a mixture of 20% phenylbis(2,4,6-trimethylbenzoyl)-phosphineoxide) and 80% 2-hydroxy-2-methyl-1-phenyl-propan-1-one.

A variety of photoinitiators can be used in the practice of thisinvention. Representative, non-limiting examples of the photoinitiatorsinclude benzophenone derivatives, acylphosphine oxide, bis-acylphosphineoxide, and α-hydroxy ketone. Representative non-limiting examples ofα-hydroxy ketones include 2-hydroxy-2-methyl-1-phenyl-propan-1-one,1-[4-2-hydroxyethoxy)-phenyl]-2-hydroxy-2-methyl-propane-1-one,1-hydroxycyclohexylphenylketone, camphorquinone, and combinationsthereof. Bisacylphosphine oxides and acylphosphine oxides are well knownmaterials that are disclosed, for example, in U.S. Pat. Nos. 4,737,593;4,792,632; 5,399,770; 5,472,992; and 6,486,228. A representativenon-limiting example of an acylphosphine oxide isdiphenyl(2,4,6-trimethylbenzoyl)phosphine oxide. A representativenon-limiting example of a bisacylphosphine oxide isphenylbis(2,4,6-trimethylbenzoyl)-phenylphosphineoxide. Combinations ofbisacylphosphine oxide and acylphosphine oxides can be employed, such asa combination of diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide andphenylbis(2,4,6-trimethylbenzoyl)-phenylphosphineoxide. It is importantthat the photoinitiator be capable of facilitating the UV curing throughthe one or more layers of composition, which can be readily determinedby one of skill in the art.

In one embodiment of this invention, the photoinitiator isbis-acylphosphine oxide, α-hydroxy ketone, or a mixture thereof.

In the absence of fiberglass, a curable formulation will typically haveabout 20 to about 70 percent of one or more acrylic oligomer, about 30to about 80 percent of one or more acrylic monomers, 0.5 to about 3percent of one or more photoinitiators, or combination thereof. In theabsence of fiberglass, a curable formulation will typically have about20 to about 70 percent of one or more acrylic oligomer, about 30 toabout 80 percent of two or more acrylic monomers, 0.5 to about 3 percentof two or more photoinitiators such as a mixture of bis-acylphosphineoxide and alpha hydroxy ketone, or combination thereof. In oneembodiment, the formulation may contain about 25 to about 60 percent ofone or more acrylic oligomer, about 40 to about 75 percent of one ormore acrylic monomer, and 0.5 to about 3 percent of one or morephotoinitiator. In one embodiment, the formulation may contain about 25to about 60 percent of one or more acrylic oligomer, about 40 to about75 percent of one or more acrylic monomers, and 0.5 to about 3 percentof two or more photoinitiators.

Fiberglass

Fiberglass may be included in the UV curable compositions of thisinvention. The fiberglass may be incorporated a variety of ways. Forexample, fiberglass particles (or fibers, or fiberglass in a variety ofshapes and sizes, including unidirectional or fabric) can be admixedwith a resin formulation to provide a heterogeneous mixture of oligomer,monomer, photoinitiator, and fiberglass particles.

Quartz filler or fabric may also be used in addition to or as analternative to the fiberglass.

Typically, alternating layers of resin and woven fiberglass fabrics areemployed. In this embodiment, a structure that may be considered to bemultilayered can be formed by applying a layer of resin to a wovenfiberglass layer, placing another layer of woven fiberglass on the resinlayer, applying a layer of resin to the second fiberglass layer, and soon. In this way, a multilayered structure with alternating resin andfiberglass layers are built up. It should be appreciated that thefiberglass layer becomes wetted out, thus there may not be discretelayers per se. Using glass fabric as opposed to other glassreinforcement insures the highest weight to strength ratio possible forthe resultant laminate. Glass and quartz fabric reinforcements permitgood transmission of the ultraviolet radiation needed for free radicalinitiation. The number of such layers employed may vary depending on theintended end us, size of the overall composition, and so on. After thedesired numbers of layers are built up, the composition that nowcontains fiberglass and curable resin (the UV curable formulation) canbe irradiated with UV light to effect curing. Owing to thephotoinitiators used in the practice of this invention, the structurecan thereby be cured. The time required to effect curing may varydepending on a variety of factors such as amount of resin, layers ofresin, temperature, type of formulation, strength and type of the UVlight source or wavelengths, and so on. In general, the time required toeffect curing may be less than one hour, typically less than 30 minutes,and often in about 20 minutes. This time is dramatically less than thetime required for the widely used thermally curable materials employedtoday for aircraft composite repair.

When the UV curable composition is used to repair composite materialssuch as in some modern aircraft wings and exterior skin, the UV curablecomposition is typically subjected to curing in a “vacuum bagging”procedure. In this regard, the composition is covered with plastic(typically on one side only) and a vacuum is pulled on the bag. In thisway a vacuum is maintained over at least one surface of the composition.The part being bagged is subjected to a compressive force that minimizesvoids. This facilitates the composition to be cured with minimalproduction of voids in the cured product. Such bagging procedures arewell known to those skilled in the art of composite repair, particularlyfor composite repair of airplanes.

Several representative woven glass fibers as well as their weaves andsizings are shown below in Table 4.

TABLE 4 Woven Glass Fiber obtained for Composite Formulation Glass Fiber# Sizing Source 7500 Abaris 1581 627 (proprietary silane) Abaris 7781497A (proprietary silane) BGF 7781 627 (proprietary silane) BGF 120 497A(proprietary silane) BGF 120 627 (proprietary silane) BGF

The weave of the fiber is a factor in the wetability of the resin, itsdrapeability, as well as a determiner in the penetrability of theultraviolet light, affecting the curing of the resin. Fiberglass with a1581 or 7781 satin weave provided a tight weave (57×54 yarns per inchcount for both types) and sufficient thickness (0.0099″ for 1581 and0.0089″ for 7781). The construction is specified as, warp ECG(electrical glass, continuous filament, filament diameter of 3.6×10⁻³inches) 1501/2 with a breaking strength of 198 lbs./inch and in the filldirection or roll width ECG 1501/2 with a breaking strength of 175lbs./inch. The fiberglass may be employed in an amount of from about 20to about 80 percent by weight, in one embodiment from about 50 to about70 percent by weight, based on the final total weight of the compositionincluding the fiberglass.

A typical UV curable composition of this invention may include about 10to about 50 percent by weight of one or more oligomers, about 20 toabout 60 percent by weight of one or more monomers, about 0.5 to about 3percent by weight of one or more photoinitiators, from about 25 to about75 percent by weight of fiberglass, or combination thereof.

Ultraviolet Equipment and Measuring Devices

This invention may use an ultraviolet light source (such as HonleUVASPOT 400/T) as well as a radiometer (such as EIT Powermap) with whichto measure the transmittance of the UV light through the samplecomposite to aid in maximizing the cure rate and percent cure. SuitableUV sources may also include those manufactured by Phillips Corporation,HPM high pressure halide lamps, HPA medium pressure metal halide lamps,HPR high pressure mercury vapor lamps, generally having a wavelength of300 to 450 nanometers (nm). A chamber may be constructed out of UVabsorbing Plexiglas to protect observers from UV radiation. Theintensity of the UV light can be adjusted by adjusting the height of thelamp above the sample within the chamber.

Cured Compositions

The cured compositions of this invention have a T_(g) above 150° C.,typically have a T_(g) above 155° C., and in one embodiment have a T_(g)above 175° C. The T_(g) of the laminate was determined using dynamicmechanical analysis and the Tg identified as the peak of the tan deltaat a frequency of 1 hertz, ASTM E1640. The cured compositions of thisinvention may be characterized as having an elastic modulus generallygreater than 2,000 psi, more typically greater than 2,500 psi, and inone embodiment greater than 3,000 psi, as determined by a four-pointbend on an Instron instrument according to ASTM D6272. The curedcompositions of this invention typically have the water absorptions lessthan 0.5 percent, and in one embodiment less than 0.3 percent, asdetermined using ASTM D570.

Additional Components

The composition of this invention may also include a variety ofadditional filler materials, which may impart additional structuralintegrity to the cured composition or to add some other property.Representative non-limiting examples of such fillers include inorganicfillers such as quartz, glass, silica, talc, carbon black, gypsum, metaloxides, calcium carbonate, and the like.

Depending on compatibility, the composition may include minor amountsof, or be devoid of, other components, such as but no limited to lightstabilizers, antioxidants, pigments, and so on.

The following examples illustrate the instant invention but are notintended to limit the scope of the invention or claims thereof. Unlessindicated otherwise, all percentages are by weight. The formulations inthe examples below have excellent adhesion to a variety of substratesand are free of hazardous air pollutants.

Representative UV Cure Resins

Certain UV resin formulations were prepared to acquire enough data fromobservations and physical testing. The developmental formulations areshown in Tables 5A and 5B, and the corresponding test results are shownin Table 5C.

TABLE 5A 5A 5B 5C 5D 5E 5F COMPONENT wt % wt % wt % wt % wt % wt % CN15117.1 43.2 40.1 45.7 45.0 SR368 (ISOTRI) 10.2 16.6 14.2 26.8 30.3 18.0Isobornyl 23.9 41.2 20.8 18.0 methacrylate Methyl methacrylateMethacrylic acid Genomer 4302 23.9 ACMO Genomer 1456 Genomer 1343Genomer 1223 23.9 41.2 20.8 32.1 23.0 18.0 Initiator (3:1 1.0 1.0 1.01.0 1.0 1.0 184/819)

TABLE 5B 5G 5H 5I 5J 5K 5L COMPONENT wt % wt % wt % wt % wt % wt % CN15150.1 51.0 57.7 57.7 55.7 SR368 (ISOTRI) 13.2 20.5 20.8 19.5 Isobornyl28.5 methacrylate Methyl methacrylate Methacrylic acid 11.7 11.7 11.711.1 Genomer 4302 41.9 ACMO 13.6 Genomer 1456 16.5 Genomer 1343 16.512.7 Genomer 1223 15.4 16.7 13.6 13.1 13.1 Initiator (3:1 1.0 1.0 1.01.0 1.0 1.0 184/819)

TABLE 5C Formulations used to develop design of experiment matrix Stress@ SAMPLE Fiberglass # Plies Tg Peaks 5% Strain 5A 1581 10 140 1-shoulder5B 1581  7 177 2 5C 1581 15 150 1-shoulder 5D 1581 15 164 1-broad 41505E 1581 15 165 1-broad 3677 5F 1581 15 165 1-broad 4267 5G 1581 15 1301-broad 4124 5H 1581 15 176 1-broad 5855 5I  7781 (497A) 10 168 1-broad3790 5J  7781 (497A) 10 170 1-shoulder 3893 5K 7781 (497A) 10 1671-broad 3933 5L 7781 (497A) 10 169 1-shoulder 3459

Typical lay-ups constructed for testing were ten glass plies thick,corresponding to a thickness of 0.11-0.19 in (2.8-4.8 mm), dependent onthe viscosity of the resin.

Cure of these composite samples is complete after twenty minutes ofexposure to ultraviolet radiation under vacuum. The vacuuming of thecomposite sample removes oxygen from the system that can inhibit thefree radical polymerization process and result in incomplete cure.

After cure is complete (20 minutes), the sample is removed from thevacuum bag and cut into pieces for physical testing.

Experimental Design Optimization

An experimental design matrix was constructed using the followingcomponents: CN151, SR368, methacrylic acid, methyl methacrylate,isobornyl methacrylate, and initiator (3:1 Irgacure 184:819). Theresponses to be tested were flexural modulus, glass transitiontemperature, water absorption, peak tan delta, and the stress at 5%strain.

These candidate composite resin formulations were then used to constructan optimized UV curable resin for aircraft repair. Below are the designsummary and the data entry sheet for the series of designed experimentsusing Degin Expert software version 6.0 to develop and optimize theformulations.

TABLE 6 Design Summary Design Summary Study Type Mixture Experiments 14Initial D-optimal Blocks No Blocks Design Design Quadratic Model Mini-Maxi- Response Name Units Observation mum mum Trans Y1 Flexural psi 0 NoData No Data None Modulus Y2 Tg 0 No Data No Data None Y3 Water % 0 NoData No Data None Absorption Y4 Peak 0 No Data No Data None Tan Delta Y5Stress psi 0 No Data No Data None at 5% Strain Com- Low High Low ponentName Units Type Actual Actual Coded A Oligomer Wt. % Mixture 30 79.5 0 BMonomers Wt. % Mixture 20 68 0 C Photoinitiators Wt. % Mixture 0.5 3 0Total = 100

TABLE 7 Data Entry Sheet for Designed Experiments Run #1 Run #2 Run #3Run #4 Run #5 Run #6 Run #7 Block Block 1 Block 1 Block 1 Block 1 Block1 Block 1 Block 1 Oligomer 55.5 Wt. % 64.55 Wt. % 78.25 Wt. % 31.5 Wt. %77 Wt. % 31.5 Wt. % 77 Wt. % Monomers 44 Wt. % 34.3 Wt. % 20 Wt. % 68Wt. % 20 Wt. % 68 Wt. % 20 Wt. % Photoinitiators 0.5 Wt. % 1.15 Wt. %1.75 Wt. % 0.5 Wt. % 3 Wt. % 0.5 Wt. % 3 Wt. % Flexural psi psi psi psipsi psi psi Modulus Tg ° C. ° C. ° C. ° C. ° C. ° C. ° C. Water % % % %% % % Absorption Peak Tan Delta Stress at 5% psi psi psi psi psi psi psiStrain Run Run Run Run Run Run #8 Run #9 #10 #11 #12 #13 #14 Block Block1 Block 1 Block 1 Block 1 Block 1 Block 1 Block 1 Oligomer 52.55 Wt. %30 Wt. % 39.8 Wt. % 79.5 Wt. % 79.5 Wt. % 49.6 Wt. % 30 Wt. % Monomers46.3 Wt. % 67 Wt. % 57.8 Wt. % 20 Wt. % 20 Wt. % 48.6 Wt. % 67 Wt. %Photoinitiators 1.15 Wt. % 3 Wt. % 2.4 Wt. % 0.5 Wt. % 0.5 Wt. % 1.8 Wt.% 3 Wt. % Flexural psi psi psi psi psi psi psi Modulus Tg ° C. ° C. ° C.° C. ° C. ° C. ° C. Water % % % % % % % Absorption Peak Tan Delta Stressat 5% psi psi psi psi psi psi psi Strain

Ten formulations, shown in Tables 8 and 9, were developed based on theoptimization of the resin formulation.

TABLE 8 Formulations from first matrix 8A 8B 8C 8D 8E COMPONENT weight %Weight % weight % weight % weight % CN151 55.50 64.55 78.25 31.50 77.00SR368 (ISOTRI) 17.60 13.72 8.00 27.20 8.00 Isobornyl methacrylate (ISBM)11.00 8.58 5.00 17.00 5.00 Methyl methacrylate (MMA) 6.60 5.15 3.0010.20 3.00 Methacrylic acid (MA) 8.80 6.86 4.00 13.60 4.00 Initiator(3:1 184/819) 0.50 1.15 1.75 0.50 3.00 Total 100.00 100.00 100.00 100.00100.00

TABLE 9 Formulations from first matrix 9A 9B 9C 9D 9E COMPONENT weight %weight % weight % weight % weight % CN151 52.55 30.00 39.80 79.50 49.60SR368 (ISOTRI) 18.52 26.80 23.12 8.00 19.44 Isobornyl methacrylate(ISBM) 11.58 16.75 14.45 5.00 12.15 Methyl methacrylate (MMA) 6.95 10.058.67 3.00 7.29 Methacrylic acid (MA) 9.26 13.40 11.56 4.00 9.72Initiator (3:1 184/819) 1.15 3.00 2.40 0.50 1.80 Total 100.00 100.00100.00 100.00 100.00

Following physical testing of the ten formulations, a secondexperimental design matrix was constructed using identical parameters tothose used in the original matrix. The components included CN151, SR368,methacrylic acid, methyl methacrylate, isobornyl methacrylate, andinitiator (3:1 Irgacure 184:819). The responses to be tested wereflexural modulus, glass transition temperature, water absorption, peaktan delta, and the stress at 5% strain. The eight formulations from thisdesign are shown below in Tables 10a and 10b.

TABLE 10a 10A 10B 10C 10D COMPONENT Weight % Weight % Weight % Weight %CN151 30.00 31.21 47.89 58.37 SR368 (ISOTRI) 26.80 27.20 20.65 16.45Isobornyl methacrylate 16.75 17.00 12.90 10.28 (ISBM) Methylmethacrylate 10.05 10.20 7.74 6.17 (MMA) Methacrylic acid (MA) 13.4013.60 10.32 8.23 Initiator (3:1 184/819) 3.00 0.79 0.50 0.50 Total100.00 100.00 100.00 100.00

TABLE 10b 11A 11B 11C 11D COMPONENT Weight % Weight % Weight % Weight %CN151 54.79 51.14 48.11 54.79 SR368 (ISOTRI) 17.88 19.34 20.56 17.88Isobornyl methacrylate 11.18 12.09 12.85 11.18 (ISBM) Methylmethacrylate 6.71 7.25 7.71 6.71 (MMA) Methacrylic acid (MA) 8.94 9.6710.28 8.94 Initiator (3:1 184/819) 0.50 0.50 0.50 0.50 Total 100.00100.00 100.00 100.00

These formulations were subjected to physical tests.

Physical Testing

Testing the UV-curable composite samples included determination ofpercent resin, determination of glass transition temperature, waterabsorption, four-point bending for strength analysis, and measuring curetemperatures. The glass fiber used in these composites was the 7781weave with a 497A sizing supplied by BFG Industries. All compositesamples for testing were constructed out of 10 plies of the glass fiber.Two commercial epoxies were also obtained for select comparison testing(Vantico 52A/B and Vantico 50A/9449 resin systems). The 50A/9449 systemwas cured at room temperature for 24 hours, followed by 2 hrs at 150° F.for maximum Tg. The 52A/B system was cured for 2 hours at 200° F.

TABLE 11 Stress @ 5% % Water Sample % Resin Tg (° C.) Strain (psi)Absorption 8A 55.4 170 2907 0.24 8B 55.0 159 3653 0.31 8C 52.9 146 29220.29 8D 41.3 175 2654 0.26 8E 49.3 131 2472 0.31 9A 41.5 169 3335 0.269B 41.1 179 2521 0.14 9C 47.3 174 3502 0.22 9D 43.7 138 2245 0.18 9E46.2 168 3003 0.27 Vantico — 124 3107 — 52A/B Vantico — 80 2966 —50A/9449

Percent Resin

Composite samples were burned at 600° C. in a muffle furnace for 1 hourin order to burn off the resin and determine the percentage of thecomponents. The differences in the percents resin are dependent on therelative viscosity of the resin formulations, excess resin with a higherviscosity is less readily removed from the wetted glass fabric, usuallyresulting in a “build-up” of resin between glass plies. The results areshown in Table 12.

TABLE 12 Percent resin in acrylic resin composites Sample % Resin 8A55.4 8B 55.0 8C 52.9 8D 41.3 8E 49.3 9A 41.5 9B 41.1 9C 47.3 9D 43.7 9E46.2

Glass Transition Temperature

Glass transition temperatures were determined using dynamic mechanicalanalysis (DMA), in which the mechanical response of the composite sampleis measured as it is deformed under periodic stress as the temperatureis elevated. The response of the sample to heating changes dramaticallyonce the T_(g) is reached and the sample softens. Typical sample sizesare 50 mm×12 mm×2.5 mm.

The glass transition temperatures of the experimental design formulationcomposites are shown in Table 13, along with the commercial epoxycomposites. The glass transition temperatures of the acrylic resinformulations exceed those of both Vantico commercial resins (thecommercial Vantico resins not an embodiment of the invention), and mostof the Tg's exceed 150° C.

TABLE 13 Glass transition temperatures Number of Peaks in Sample Tg (°C.) DMA Plot 8A 170 2 8B 159 2 8C 146 2 8D 175 2 8E 131 2 9A 169 2 9B179 2 9C 174 2 9D 138 2 9E 168 2 Vantico 124 1 52A/B Vantico 80 250A/9449

Stress @ 5% Strain

The stress at 5% strain was determined by four-point bend on an Instroninstrument according to ASTM D6272. The stresses for the experimentaldesign formulation composites are shown in the table below along withthose observed for two commercial products: Vantico 50A/B and 52A/9449.Several of the resin formulations showed either comparable or greaterresistance to elevated stresses than the example commercial epoxiescurrently in use.

TABLE 14 Stress @ 5% strain (4-point bend) for acrylate and Vanticoresin composites Stress @ 5% Sample Strain (psi) 8A 2907 8B 3653 8C 29228D 2654 8E 2472 9A 3335 9B 2521 9C 3502 9D 2245 9E 3003 Vantico 310752A/B Vantico 2966 50A/9449

Water Absorption

Due to fact that some of the monomers and oligomers used in the resinformulations are hydrophilic in nature, the water absorptions of thecomposites were determined using ASTM D570. Samples were weighed beforeand after complete immersion in distilled water for 24 hours at roomtemperature. Percent water absorption is presented below in Table 15.

TABLE 15 Water absorption data for acrylic resin composites Sample %Water Absorption 8A 0.24 8B 0.31 8C 0.29 8D 0.26 8E 0.31 9A 0.26 9B 0.149C 0.22 9D 0.18 9E 0.27

Cure Temperature

The cure temperature of a composite was measured underneath a 10-plysample using a thermocouple. The temperature was recorded beforeexposure to the UV lamp, every five seconds after initial exposure, andthen less frequently as the changes in temperature with time weresmaller. A plot of temperature versus time showed that a low exotherm ofless than 100° C. occurred, with the peak temperature of ˜60° C. wasreached at 35-40 seconds.

A study was done to determine the effects of temperature extremes on thecuring of the acrylate resins. A 0.1825″ thick sample was prepared andheated to 60° C. (140° F.) then exposed to ultraviolet radiation. Curewas accomplished within 2 minutes.

At the other temperature extreme, a 0.1825″ thick sample was preparedand cooled to −30° C. (−22° F.) and exposed to ultraviolet radiation.The sample was kept in a −30° C. (−22° F.) atmosphere by suspension overliquid nitrogen. Despite the frigid air around the sample and theinitial temperature of the sample itself, the temperature of the samplebecame very hot within 30 seconds of UV exposure as the reaction wasinitiated. Complete cure was observed within two minutes. Apparently,neither hot nor cold temperatures have any appreciable effect on thecure of the resin systems.

Based upon the results of the physical testing of the second round ofeight formulations, four formulations were chosen to be used insimulated aircraft repairs. The formulations are shown in Table 16, andTable 17 outlines the pertinent physical data.

TABLE 16 16A 16B 16C 16D COMPONENT Weight % Weight % Weight % Weight %CN151 30.00 31.21 47.89 54.79 SR368 (ISOTRI) 26.80 27.20 20.65 17.88Isobornyl methacrylate 16.75 17.00 12.90 11.18 (ISBM) Methylmethacrylate 10.05 10.20 7.74 6.71 (MMA) Methacrylic acid (MA) 13.4013.60 10.32 8.94 Initiator (3:1 184/819) 3.00 0.79 0.50 0.50 Total100.00 100.00 100.00 100.00

TABLE 17 Physical properties of the four formulations from Table 16 TgNumber of Peaks Stress @ 5% % Water Sample (° C.) in DMA Plot % ResinStrain (psi) Absorption 16A 173 2 31.9 1784 0.39 16B 176 2 37.4 19660.40 16C 165 1 49.1 2721 0.28 (shoulder) 16D 157 2 49.3 2616 0.26

Perform and Test Composite Repairs

Although bagging schedules may vary depending on the particular resintype or the number of layers of resin-soaked glass fabric, it has beenfound that an excellent bagging schedule for the optimized formulationsmay consist of one layer of Teflon-coated fiberglass over the wetlay-up, followed by a 120 style fiberglass bleeder, a layer of P3perforated fluoropolymer, another 120 fiberglass bleeder, and a Nylonbarrier layer topped with a 120 fiberglass layer as a breather.

Almost all samples were cured at 20 minutes with a Honle UVASPOT/400Tultraviolet lamp at a distance of about 9 inches. The exceptionsincluded a sample that was cured in direct sunlight for 20 minutes.However, prepreg composite samples were effectively used in a room withlarge windows, allowing for at least 20 minutes of working time withoutany appreciable cure occurring via indirect sunlight. The core of thisrepair had 11 plies of composite and appeared to cure thoroughly fromvisual inspection.

Kevlar fabric was also used in performing a wet lay-up with one of theresins. Following the standard twenty-minute cure, only the top of theuppermost of the four layers cured. No cure occurred below the toplayer.

A composite repair was also attempted on a leading edge, a morechallenging geometry, using the standard bagging schedule and cure time.The cure was effective over the full area of the repair, around theentire diameter of the curved repair.

Further modifications and alternative embodiments of this invention willbe apparent to those skilled in the art in view of this description.Accordingly, this description is to be construed as illustrative onlyand is for the purpose of teaching those skilled in the art the mannerof carrying out the invention. It is to be understood that the forms ofthe invention herein shown and described are to be taken as illustrativeembodiments. Equivalent elements or materials may be substituted forthose illustrated and described herein, and certain features of theinvention may be utilized independently of the use of other features,all as would be apparent to one skilled in the art after having thebenefit of this description of the invention.

1. An ultraviolet light curable composition useful for repairingcomposite materials, comprising: an acrylic oligomer, an acrylicmonomer, a photoinitiator, and fiberglass, wherein the photoinitiator isa combination of a bis-acylphosphine oxide and an alpha hydroxy ketone,and wherein the bis-acylphosphine oxide to alpha hydroxy ketone ratio isfrom about 1:4 to about 4:1.
 2. The composition of claim 1, wherein theacrylic oligomer is an epoxy acrylate, urethane acrylate, polyesteracrylate, polyether acrylate, amine modified polyether acrylate, acrylicacrylate, or combination thereof.
 3. The composition of claim 1, whereinthe acrylic oligomer is an epoxy acrylate.
 4. The composition of claim1, comprising two or more oligomers, two or more acrylic monomers, twoor more photoinitiators, or combination thereof.
 5. The composition ofclaim 1, wherein the acrylic monomer is selected from the groupconsisting of methyl methacrylate (MMA), ethyl methacrylate, methacrylicacid (MA), isobornyl methacrylate (ISBM), ethylene glycol dimethacrylate(EGDM), ethoxylated bisphenol A diacrylate esters (BPADAE),tetraethylene glycol dimethacrylate (TEGDM), diethylene glycoldimethacrylate (DEGDM), diethylene glycol diacrylate (DEGDA),tris(2-hydroxyethyl) isocyanurate triacrylate (ISOTRI); a diacrylate, analkyl or hydroxy alkyl esters of acrylic acid; a diacrylate, an alkyl orhydroxy alkyl esters of methacrylic acid; butyleneglycol diacrylate andtriacrylate, 1,6-hexanediol diacrylate, tetraethyleneglycol diacrylateand triacrylate, polyethylene glycol diacrylate and triacrylate,bisphenol A diacrylate and triacrylate, pentaerythritol diacrylate andtriacrylate and tetraacrylate; methyl acrylate, ethyl acrylate, butylacrylate, 2-ethylhexyl acrylate, 2-hydroxyethyl acrylate, isobornylacrylate, ethylene glycol diacrylate, propylene glycol diacrylate,neopentyl glycol diacrylate, hexamethylene glycol diacrylate,4,4′-bis(2-acryloyloxyethoxy)diphenylpropane, trimethylolpropanetriacrylate, vinyl acrylate, and combinations thereof.
 6. Thecomposition of claim 1, comprising as the acrylic monomer a combinationof tris(2-hydroxyethyl)isocyanurate triacrylate, isobornyl methacrylate,methyl methacrylate, 1,6-hexanediol diacrylate, and methacrylic acid. 7.The composition of claim 1, wherein the composition comprises about 10to about 50 percent by weight of the acrylic oligomer, about 20 to about60 percent by weight of the acrylic monomer, about 0.5 to about 3percent by weight of the photoinitiator, and about 25 to about 75percent by weight of the fiberglass.
 8. The composition of claim 1,wherein the cured composition has a T_(g) greater than 150° C.